Non-volatile memory devices are used in certain applications where data must be retained when power is disconnected. Applications include general memory cards, consumer electronics (e.g., digital camera memory), automotive (e.g., electronic odometers), and industrial applications (e.g., electronic valve parameter storage). The non-volatile memories may use phase-change memory materials, i.e., materials that can be switched between a generally amorphous and a generally crystalline state, for electronic memory applications. The memory of such devices typically comprises an array of memory elements, each element defining a discrete memory location and having a volume of phase-change memory material associated with it. The structure of each memory element typically comprises a phase-change material, one or more electrodes, and one or more insulators.
One type of memory element originally developed by Energy Conversion Devices, Inc. utilizes a phase-change material that can be, in one application, switched between a structural state of generally amorphous and generally crystalline local order or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. These different structured states have different values of resistivity, and therefore each state can be determined by electrical sensing. Typical materials suitable for such application include those utilizing various chalcogenide materials. Unlike certain known devices, these electrical memory devices typically do not use field-effect transistor devices as the memory storage element. Rather, they comprise, in the electrical context, a monolithic body of thin film chalcogenide material. As a result, very little area is required to store a bit of information, thereby providing for inherently high-density memory chips.
The state change materials are also non-volatile in that, when set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance value, that value is retained until reprogrammed as that value represents a physical state of the material (e.g., crystalline or amorphous). Further, reprogramming requires energy to be provided and dissipated in the device. Thus, phase-change memory materials represent a significant improvement in non-volatile memory technology.
However, current phase-change memory devices incur energy losses in the form of heat dissipation through adjacent and intrinsic structures, reducing the efficiency of the memory device. This means that current requirements for programming are higher than need be. when there is heat loss.
In addition to the aforementioned problems, the use of multi-level storage (representation of multiple bits within one physical memory cell) requires predictable and configurable programming characteristics that are not realized with some current devices. Further, current devices do not allow for direct imaging, measurement, or optical programming of the memory device structures that would allow for improved research and development, as well as novel new device design and product applications. Also, current devices are limited to memory applications.
Thus, a need has arisen to improve the efficiency of the memory device relating to the containment of heat resulting in reduction of necessary programming current. Additionally, it is desirable to reduce the number of process steps required to produce the memory device in order to increase yield.
Further, it is desirable to provide a memory device having improved controllability of programming for multi-level storage applications. A further need also exists to image, directly measure, and/or characterize the memory device during and after programming operations. It is also desirable to expand the range of uses for phase-change devices, as well as other novel optical devices.